1. Field of the Invention
The present invention relates to performing downlink synchronization when some cells are on/off under an environment in which a macro cell and a small-scale cell coexist.
2. Related Art
3GPP (3rd Generation Partnership Project) LTE (Long Term Evolution) that is an advancement of UMTS (Universal Mobile Telecommunication System) is being introduced with 3GPP release 8. In 3GPP LTE, OFDMA (orthogonal frequency division multiple access) is used for downlink, and SC-FDMA (single carrier-frequency division multiple access) is used for uplink. To understand OFDMA, OFDM should be known. OFDM may attenuate inter-symbol interference with low complexity and is in use. OFDM converts data serially input into N parallel data pieces and carries the data pieces over N orthogonal sub-carriers. The sub-carriers maintain orthogonality in view of frequency. Meanwhile, OFDMA refers to a multiple access scheme that realizes multiple access by independently providing each user with some of sub-carriers available in the system that adopts OFDM as its modulation scheme.
Recently, 3GPP LTE-Advanced (LTE-A) which is an evolution of 3GPP LTE has been discussed.
In addition, a hetero-network in which a macro cell and a small-scale cell coexist has been discussed recently. Particularly, discussions have been progressed in order to detour traffic by dispersing terminals connected to a macro cell into a small-scale cell.
However, small-scale cells may be unintentionally deployed with high density, and as a result, an inter-cell interference problem may occur. In order to solve the inter-cell interference problem, a concept that arbitrarily turns on/off some small-scale cells may be considered. When the small-scale cells are turned on/off as described above, a problem that a UE may not normally perform downlink synchronization may occur.